Long haired Chihuahuas can be pretty darn fluffy all on their own. So Pom/Chi is definitely believable IMO. His head and neck structure looks a little odd so maybe some other breed in there or maybe just the angle he is looking when the photo was taken.
A lot of Pom and long-haired Chi, I agree! For some reason Tibetan Spaniel came to mind with that tail, but they're not exactly thick on the ground so that's probably just a coincidence, hah. I could see another spitzy breed in there to account for the heavier head, or he could be from a more deer-headed chihuahua and/or one of the more fox-faced pomeranians. Neither are the typical "show" look, but are quite common in the breeds.
He's a cutie, though, and looks like a real charmer!